It has been known for some time that under certain conditions shape-maintaining pulses of electromagnetic radiation can exist in single mode optical fiber. Such pulses are termed solitons (A. Hasegawa et al, Applied Physics Letters, Volume 23(3), pages 142-144, 1973). The existence of solitons has been experimentally demonstrated (L. F. Mollenauer et al, Physical Review Letters, Volume 45(13), pages 1095-1098, 1980), and their utility for high capacity communications systems has been disclosed (co-assigned U.S. Pat. No. 4,406,516, and A. Hasegawa et al, Proceedings of the IEEE, Volume 69(9), pages 1145-1150 (1981), both incorporated herein by reference).
Furthermore, it has been found that solitons can be amplified nonelectronically without loss of soliton character (A. Hasegawa, Applied Optics, Volume 23(19), pages 3302-3309 (1984), and co-assigned U.S. Pat. No. 4,558,921, both incorporated herein by reference). Nonelectronic amplification, e.g., by means of the Raman effect, increases the pulse height and decreases the pulse width of fundamental soliton pulses. This coupling between pulse height and pulse width is an attribute of solitons, and its existence has been experimentally verified in single mode fiber with loss compensated by Raman gain (L. F. Mollenauer et al, Optics Letters, Volume 10, pages 229-231, 1985).
Although prior art soliton communication systems potentially have exceedingly high bandwidth, there is some uncertainty amongst workers is the field concerning the actually realizable bandwidth, due to interaction between soliton pulses. For instance, in a very recent paper (P. L. Chu et al, Electronics Letters, Volume 21(24), pages 1133-1134, November 1985), it is stated that, before soliton communications systems can become practical, the problem of the mutual interaction among solitons has to be solved. It is also stated there that such interaction can result is bandwidth reduction by a factor of 10, due to the variation of soliton separation as the solitons propagate down the fiber. As a solution to the alleged interaction problem, that paper teaches the use of solitons of unequal amplitude.
The potential bandwidth of a prior art soliton fiber communication system is so high that the actual rate of information transmission of such systems very likely will be limited by the bandwidth of available sources and detectors. Furthermore, the use of very high speed sources and detectors can be expected to add to the cost of such systems, and possibly adversely impact reliability. For these and other reasons, a soliton communication system that has as much or more bandwidth than prior art soliton systems but can fully utilize the available bandwidth without requiring unusually high speed sources, detectors and other electronics to achieve this end, would clearly be of great interest. This application discloses such a system.